Re-implementation of rustbootd's ecat_el6695_* examples as a single binary
with fixes found in review and on hardware:
- DC-follow PLL hardened against period-2 hunting: slew-limited anchor
(+/-50us/cycle), bistable-trap snap re-anchor, re-prime on stale deadline
- drift-free absolute-grid ticker mode; probe mode for timestamp forensics
- bounded-memory online stats (histograms), graceful SIGINT/SIGTERM shutdown
with full report, error-streak abort
- timestamp plausibility filter comparing against the previous raw sample
(avoids the deadlock after a startup outlier)
- XFC scope waveform options: --el2202 (with --el2202-dual), --el2262,
--el1252 latch timestamp readback with per-channel edge statistics
- register access unified in regs.rs: named bit constants everywhere,
read-modify-write for enable/activation bytes
- vendored patched ethercrab 0.7.1 (sdo_write_complete, send_raw_coe)
Verified on J1900 (PREEMPT_RT 6.6.135): 600k cycles/600s exact 1 kHz,
tx/rx zero errors, phase_err p50=164us std=5us; EL2202<->EL1252 loopback
edge interval mean 2000.24us std=24.65us.
